1. Field of the Invention
This invention relates to a magnetic recording sheet, such as a magnetic recording disc or the like, and a method making such a sheet.
2. Description of the Prior Art
As is well known, a magnetic recording sheet comprises a flexible film base coated with a magnetic layer and is generally of a circular shape with a center opening for engagement with a rotatable member of a driving means. The magnetic layer generally contains a plurality of individual magnetic particles such as r-Fe.sub.2 O.sub.3. Signals are recorded on the magnetic layer of the magnetic recording sheet along concentric circles or a spiral line. Accordingly, the magnetic particles carried on the magnetic recording sheet are preferably oriented in concentric circles for the sake of lowering output fluctuation and obtaining a higher output level.
In manufacturing such a circular magnetic recording sheet, the magnetic particles are applied to a continuous web of the support film base in a coating mixed with a binder substance and then the individual circular magnetic recording sheets are stamped out from the continuous web after the coating is solidified. As is well known in the art, the particle orientation must be fixed before the coating is solidified and, consequently, before stamping out the magnetic recording sheets.
Therefore, if the magnetic particles are oriented along concentric circles, the center of the individual sheets must be made to coincide with the common center of the concentric circles when the circular magnetic recording sheet is being stamped out. This requirement entails manufacturing difficulties and limits the possibility of mass production of circular magnetic recording sheets with a concentric magnetic particle orientation. In fact, it is impossible to massproduce such magnetic recording sheets.
Various methods have been proposed for producing magnetic recording sheets having unique particle orientations which can be mass-produced more easily, while at the same time lowering output fluctuation and obtaining a higher output level.
For example, in Japanese Patent Public Disclosure No. 14307/1975, there is disclosed a magnetic recording sheet in which the magnetic layer consists of three zones which are arranged in parallel in the direction parallel to a diameter of the sheet. The intermediate zone interposed between the other two zones is twice as wide as one of the other two zones and includes the center of the magnetic recording sheet at the center of its width.
In the intermediate zone, the magnetic particles are oriented in the longitudinal direction of the zone, and in the other two zones, the magnetic particles are oriented in the direction substantially perpendicular to the longitudinal direction of the zone.
When producing this type of magnetic recording sheet, a coating including magnetic particles mixed with a binder substance is applied to a continuous web of the support film base along its longitudinal direction. The magnetic particles included in the intermediate zone of the web are oriented in the longitudinal direction of the web, and the magnetic particles included in opposite sides zone of the web are oriented in the direction perpendicular to the longitudinal direction of the web. Then individual sheets are stamped out from the web in such a manner that the center of the sheet coincides with the longitudinal axis of the web of the intermediate zone.
It is apparent that the magnetic recording sheet of this type is more suitable for mass production than that having a concentric particle orientation, since the center of the sheet is only required to coincide with the longitudinal axis of the web. However, not only is it still rather difficult to mass-produce this type of magnetic recording sheet, but the output fluctuation thereof is also much larger than that of the magnetic recording sheet having a concentric particle orientation.
In more detail, in accordance with the magnetic recording sheet of the above described type as disclosed in Japanese Patent Public Disclosure No. 14307/1975, the output fluctuation which has been as large as 3dB can be reduced to about 1.0dB. This level of output fluctuation is still desired to be much more reduced.